Help with circuit mod - Adding a power tube

[MadAsAHatter]

I just purchased a MOD 102+ amp kit. It's supposed to be Fender Champ-ish (at least in tone) but uses an EL84 instead of a 6V6. When researching the kit before buying I came across a video from Dlabs where he had modded it to use a 6V6 instead. I saw a few other forum posts on this too and even one where it was being modded to have both and use a toggle to switch between the two. The problem is no one really explained exactly what they did.

I'm wanting to do something similar. I'd like to add an octal socket to have the option to use either EL84 or 6V6. But unlike the one person, not bother with the toggle switch and do it similar to how my N5X kit is done. In the N5X you put one tube or the other in, not both at the same time. Using the schematics from both amps I've combined them to something that I think would work.

My questions are:

1. Most importantly, would this actually work?
2. What value cap and resistor would I use coming off pin 1/8 on the 6v6 socket?
3. How would that lay out physically; specifically where would I connect the resistor/capacitor that come off of the 6v6 socket pins 1/8? This kit uses tag strips.

This is the original schematic of the MOD 102+

This is the modification I worked up to add the octal socket

The original physical layout (heater wires aren't shown in this pic)

 

[glpg80]

You’ll need a new OT. The primary impedance most certainly will not be 5k anymore. I don’t remember a 6V6 off the top of my head. You’d be better off putting another EL84 in it which also reduces the primary impedance to 2.5k on a 5k tapped OT, which means you’d also reduce the secondary impedance by an exact safe factor of two and always using a 4 ohm load on the output instead of the 8 ohm. If you use a 6V6 you’d get some weird non-standard output requirement that no speaker has which is why you need a new OT.

The resistor in question is for proper cathode bias and is a value chosen for proper class A spec. You’re better off also installing a phase inverter and running class A/B but by that point you’ve built the back end of a new amp.

The number of components needed to do this also doubles, you’ve over simplified it in your drawing. Each tube should get its own grid stopper resistor for stability and each should also get its own screen grid resistor in case a tube shorts. You’ll be drawing double the current so you need to swap all of the resistors in the b+ supply to double the wattage rating to prevent a fire hazard.

Designing amps is no joke and building them is also a pain. To do this properly and maintain value and safety for yourself and someone else if you ever sell it, there is a monumental undertaking to adding a power tube to an amp.

 

[MadAsAHatter]

You’ll need a new OT. The primary impedance most certainly will not be 5k anymore. I don’t remember a 6V6 off the top of my head. You’d be better off putting another EL84 in it which also reduces the primary impedance to 2.5k on a 5k tapped OT, which means you’d also reduce the secondary impedance by an exact safe factor of two and always using a 4 ohm load on the output instead of the 8 ohm. If you use a 6V6 you’d get some weird non-standard output requirement that no speaker has which is why you need a new OT.

The resistor in question is for proper cathode bias and is a value chosen for proper class A spec. You’re better off also installing a phase inverter and running class A/B but by that point you’ve built the back end of a new amp.

The number of components needed to do this also doubles, you’ve over simplified it in your drawing. Each tube should get its own grid stopper resistor for stability and each should also get its own screen grid resistor in case a tube shorts. You’ll be drawing double the current so you need to swap all of the resistors in the b+ supply to double the wattage rating to prevent a fire hazard.

Designing amps is no joke and building them is also a pain. To do this properly and maintain value and safety for yourself and someone else if you ever sell it, there is a monumental undertaking to adding a power tube to an amp.

Thanks for the feedback. So that's pretty much a big ole NOPE, ain't gonna happen for me. I was optimistic that even if I got it wrong it could still be a somewhat straightforward thing to do. Nearly redesigning the back end isn't something I want to do with this amp and frankly well out of my range. My skills now are pretty much limited to copying established circuits and mods; assuming I have access to a wiring layout. I can follow a schematic well enough, but can't translate it into a physical layout for shit.

I'll just build it as designed. Maybe in the future I'll have gained enough skill to do more and I can circle back to this idea. Probably best I stick to my original plan anyway... Start simple as I have, and slowly build my abilities as work my way through more complex circuits. Maybe one day I'll get to the point of doing some good in depth mods & builds and can come back to a few of these ideas I'm not ready for now.


Another question related to my original topic. As I mentioned I read a few things about people converting this amp to 6V6. Most of what I read was stating that most any transformer could handle and EL84 to 6V6 change and the only thing that needed to be done was change out the socket and bias resistors. Given my limited knowledge and your explanation, I'm sure there're more to it that. For the sake of me learning something, is there any truth to things I've read? Given one knows what their doing, is a straight swap as complex as adding one like I was asking about would be?

 

[glpg80]

Thanks for the feedback. So that's pretty much a big ole NOPE, ain't gonna happen for me. I was optimistic that even if I got it wrong it could still be a somewhat straightforward thing to do. Nearly redesigning the back end isn't something I want to do with this amp and frankly well out of my range. My skills now are pretty much limited to copying established circuits and mods; assuming I have access to a wiring layout. I can follow a schematic well enough, but can't translate it into a physical layout for shit.

I'll just build it as designed. Maybe in the future I'll have gained enough skill to do more and I can circle back to this idea. Probably best I stick to my original plan anyway... Start simple as I have, and slowly build my abilities as work my way through more complex circuits. Maybe one day I'll get to the point of doing some good in depth mods & builds and can come back to a few of these ideas I'm not ready for now.


Another question related to my original topic. As I mentioned I read a few things about people converting this amp to 6V6. Most of what I read was stating that most any transformer could handle and EL84 to 6V6 change and the only thing that needed to be done was change out the socket and bias resistors. Given my limited knowledge and your explanation, I'm sure there're more to it that. For the sake of me learning something, is there any truth to things I've read? Given one knows what their doing, is a straight swap as complex as adding one like I was asking about would be?

If what you’re asking is whether I’m blowing smoke to your project, you can do anything to an amp you want. It’s yours. Just know there’s a right way and a wrong way. I explained the overview of the right way above.

You can take my word it’s a complete gut job and start over to do correctly, or take shortcuts and corners like everyone else and cross your fingers you don’t build something dangerous. Your choice.

 

[MadAsAHatter]

If what you’re asking is whether I’m blowing smoke to your project, you can do anything to an amp you want. It’s yours. Just know there’s a right way and a wrong way. I explained the overview of the right way above.

You can take my word it’s a complete gut job and start over to do correctly, or take shortcuts and corners like everyone else and cross your fingers you don’t build something dangerous. Your choice.

Hang on, we need to back up a bit here before things start coming across in an unintended way. Your response here is coming off rather snarky. I'm going to assume you did not mean it that way. And I fully apologize if my previous post sounded like I was questioning your advice or insinuating you were blowing smoke at me.

I understand this stuff enough to know there's a crap ton of things to learn. But we all have to start somewhere. I seriously didn't know if there was a big difference between adding a tube like I originally asked and complete replacement like my follow up question. I read a few old posts on other forums about the latter, but also know not to blindly trust everything on the internet. That's why I asked. I've been here long enough to see that several; including you, have good & reliable knowledge on the subject. Even just doing this as a hobby, I really want to learn the right way to do things and not end up like the plethora of homebrew hack jobs. If anything I ask comes off as questioning the authenticity of advice, I 100% do not mean it that way. I'm legitimately asking because I don't know and want to learn the correct way.

Moving past that... I do appreciate the advice and explanation on how much would be involved in trying to convert this to 6V6. Something like that is well outside of my skill set. With this particular project I'm going to stick with building it to spec and not attempt any half baked mods. There are only 2 extra things I want to do which should be simple enough for me. One; which is shown in the supplemental instructions, is add a filter cap drain resistor as an additional safety measure. The other is to install an IEC socket instead of directly wiring the power cord.

 

[glpg80]

I want you to feel like you can ask questions. No snarky attitude intended I felt it needed to be said you can do whatever you please. It’s not my amplifier and doing a job right does mean you’ve got a lot to learn. With that said yes, what I say isn’t based on some internet post, it’s based on a deep understanding of electrical engineering.

What you need to know is that you should absolutely research how to do it right and try. This amp project teaches the same skills the big ones abide by. So it would be better to learn it now than later.